The propose of this research is to demonstrate and understand the development of individual thalamocortical fibers in their entirety from the ventroposterior medial nucleus of the thalamus to the somatosensory cortex of the rat and mouse. Individual neurons and their axons will be labeled by selective injection of various dyes in a slice preparation. We developed this preparation to capture the entire somatosensory thalamocortical pathway in a single tissue slice which may be kept in vitro for hours. Specifically a number of well filled axons will be harvested from normal animals, animals subjected to various peripheral and central lesions as neonates and ultimately in a series of developing animals. Animals will be prepared by the animal CORE. Our goal is to characterize these individual fibers from the viewpoint of competitive mechanisms that normally operate in the development of brain maps. Special attention will be given to comparisons with axons in other parts of the central trigeminal pathway as demonstrated in other projects in this grant and to take advantage of the expertise of the other investigators to add these specific data to complementary studies of axonal populations being carried out in our laboratory.